1. Field of the Invention
This invention concerns wall thickness measurements in general. More specifically it concerns a method and apparatus for measuring wall thickness of metallic material. It is also well adapted for making combined measurements of wall conditions in pipelines and the like.
2. Description of the Prior Art
Magnetic methods have been used heretofore for finding anomalies of pipes or pipelines. But, they have not been found quantitatively accurate, and they often fail to provide a true qualitative representation of accurate wall conditions. It has been found that magnetic permeability differences between the steel pipe and surrounding medium, become a primary problem when sensor pads bounce and vibrate from the pipe wall as a result of passing weld joints and other obstructions. Also, magnetic fields induced into the pipe by current conducting brushes or permanent magnets are often unstable.
Consequently, it is an object of this invention to provide a method for measuring wall thickness of metallic materials which method uses a pulsed magnetic reluctance coil rather than passive sensor coils or diodes in the presence of a steady state magnetic field. In addition, this method provides for determining the distance between such pulsed coil and the metallic material in order that the inductance measured may be compensated to provide accurate indication of wall thickness.